1. Field of the Invention
The present invention relates to a rotation angle sensor which detects a rotation angle by amounts of changes in electrostatic capacitance.
2. Description of the Background Art
FIG. 7 shows a capacitance type rotation angle sensor 1, which is of interest to the present invention. This rotation angle sensor 1 is described in Japanese Utility Model Application Laying-Open No. 55-88109 (1980).
This sensor 1 comprises a housing 2 consisting of a metal. The housing 2 rotatably holds a shaft 5 through bearings 3 and 4. At least a portion of the shaft 5 which is positioned in the housing 2 is made of a metal to be electrically insulated with respect to the housing 2.
The housing 2 is provided therein with a plurality of fixed electrodes 6 and a plurality of rotational electrodes 7 which are positioned between the fixed electrodes 6. The rotational electrodes 7 are mounted on the shaft 5. The shaft 5 is in sliding contact with a collector 8, consisting of a piano wire, for example, which is electrically connected with an external terminal 9. The external terminal 9 passes through the housing 2 via an insulating bushing 10. The insulating bushing 10 is also adapted to hold the collector 8.
On the other hand, the fixed electrodes 6 are electrically connected to another external terminal 11. The external terminal 11 passes through the housing 2 via another insulating bushing 12. The insulating bushing 12 is also adapted to hold the fixed electrodes 6.
The shapes of the fixed electrodes 6 and the rotational electrodes 7 are so selected that overlap areas thereof are varied with changes in rotation angle of the shaft 5. The shaft 5 is rotated in response to a rotation angle to be detected. Therefore, the overlap areas of the fixed and rotational electrodes 6 and 7 are varied in correspondence to the rotation angle to be detected, so that an electrostatic capacitance which is formed by the fixed and rotational electrodes 6 and 7 is changed in response thereto. This electrostatic capacitance is derived from the external terminals 9 and 11. Thus, it is possible to recognize the rotation angle to be detected by a change of the electrostatic capacitance.
In the aforementioned sensor 1, however, conductive paths connecting the rotational electrodes 7 with the external terminal 9 include a portion of the collector 8 which is in sliding contact with the shaft 5, and hence the electrical properties and the life of this sensor 1 depend on the state of the collector 8, which is in sliding contact with the shaft 5, and the life of the collector 8 itself. Consequently, this sensor 1 is insufficient in reliability and durability.
On the other hand, FIGS. 8A and 8B show principal elements which are included in another type of capacitance type rotation angle sensor which is of interest to the present invention. This rotation angle sensor is described in Japanese Patent Application Laying-Open No. 4-172218 (1992).
As shown in FIGS. 8A and 8B, this sensor comprises first and second fixed electrodes 13 and 14 which are opposed to each other, and a rotational plate 15 consisting of a ferroelectric substance which is inserted between the fixed electrodes 13 and 14. The first fixed electrode 13 has a circular shape, while the second fixed electrode 14 is split into first and second fixed split electrode members 16 and 17 having semicircular shapes respectively. The rotational plate 15, which is in the form of a semicircle, is mounted on a rotatably held shaft 18. First, second and third external terminals 19, 20 and 21 are electrically connected to the first fixed electrode 13, the first fixed split electrode member 16 and the second fixed split electrode member 17 respectively.
In this sensor, the shaft 18 is rotated in correspondence to a rotation angle to be detected. In response to this rotation of the shaft 18, the rotational plate 15 is so rotated that an electrostatic capacitance which is formed between the first fixed electrode 13 and the first fixed split electrode member 16 and that formed between the first fixed electrode 13 and the second fixed split electrode member 17 are differentially changed and such differentially changed electrostatic capacitances are derived by the first and second external terminals 19 and 20 and the first and third external terminals 19 and 21 respectively.
The aforementioned sensor solves the problem encountered in the sensor 1 shown in FIG. 7, i.e., the problem that the electrical properties and the life of the sensor depend on the state of sliding contact and the life of the sliding contact portion, since no sliding contact portions are provided in the conductive paths for deriving the electrostatic capacitances. However, the sensor shown in FIGS. 8A and 8B has the following problem:
The electrostatic capacitance which is formed by the first fixed electrode 13 and the first fixed split electrode member 16 or by the first fixed electrode 13 and the second fixed split electrode member 17 is maximized when the rotational plate 15 completely overlaps with the first or second fixed split electrode member 16 or 17. Assuming that the rotational plate 15 is positioned at the center of the space between the firsthand second fixed electrodes 13 and 14, the maximum electrostatic capacitance C.sub.MAX is expressed as follows: EQU C.sub.MAX =.di-elect cons..sub.0 .di-elect cons..sub.S S/{(D-T).di-elect cons..sub.S +T} (1)
where D represents the space between the first and second fixed electrodes 13 and 14, T represents the thickness of the rotational plate 15, S represents the area of opposition of the first fixed electrode 13 and the first or second fixed split electrode member 16 or 17, .di-elect cons..sub.0 represents the dielectric constant of a vacuum, and .di-elect cons..sub.S represents the relative dielectric constant of the rotational plate 15.
On the other hand, the electrostatic capacitance which is formed by the first fixed electrode 13 and the first fixed split electrode member 16 is minimized when no rotational plate 15 is provided therebetween. Similarly the electrostatic capacitance which is formed by the first fixed electrode 13 and the second fixed split electrode member 17 is also minimized when no rotational plate 15 is provided therebetween. Such a minimum electrostatic capacitance C.sub.MIN is expressed as follows: EQU C.sub.MIN =.di-elect cons..sub.0 S/D (2)
Thus, the difference .DELTA.C between the maximum and minimum values C.sub.MAX and C.sub.MIN is expressed as follows: ##EQU1##
From the equation (3), it is understood that it is possible to increase the difference .DELTA.C as the space (D-T)/2 between the rotational plate 15 and the first or second fixed electrode 13 or 14 is reduced. In this case, the rotational plate 15 must be positioned at the center of the space D between the first and second fixed electrodes 13 and 14, as described above. While this condition is satisfied, however, reduction of the aforementioned space (D-T)/2 is limited. In relation to this, Japanese Patent Application Laying-Open No. 4-172218 (1992) discloses no means for adjusting the rotational plate 15 for positioning the same at the center of the space D between the first and second fixed electrodes 13 and 14.
From the equation (3), it is also understood that it is possible to increase the difference .DELTA.C as the area S of opposition is increased. If the area S of opposition is increased, however the overall sensor is undesirably increased in size. Therefore, this means cannot be simply employed. Further, while substantial increase of the minimum electrostatic capacitance is inhibited, increase of the area S of opposition is limited, because the minimum electrostatic capacitance is influenced by the area S of the fixed electrode 13 and the fixed split electrode member 16 or 17 as well as the space D between them.
While it is possible to increase the difference .DELTA.C between the maximum and minimum electrostatic capacitances by reducing the space (D-T)/2 between the rotational plate 15 and the first or second fixed electrode 13 or 14 or increasing the area S of opposition of the first fixed electrode 13 and the first or second fixed split electrode member 16 or 17 in the sensor shown in FIGS. 8A and 8B as hereinabove described, increase of the difference .DELTA.C is limited in either case. Thus, it is impossible to remarkably improve rotation angle detection sensitivity of this sensor.